Exhaust manifolds, front pipes, center pipes, and other exhaust system members of automobiles carry high temperature exhaust gas which is exhausted from the engine, so the materials forming the exhaust members are required to offer oxidation resistance, high temperature strength, heat fatigue characteristics, and diverse other characteristics.
In the past, cast iron has generally been used for automobile exhaust members, but from the viewpoint of the toughening of exhaust gas regulations, improvement of engine performance, reduction of the weight of chasses, etc., stainless steel exhaust manifolds are being used.
The temperature of exhaust gas differs depending on the vehicle type and engine structure, but often becomes 600 to 800° C. or so. In environments of long term use in such a temperature region, materials which have an excellent high temperature strength and oxidation resistance are being demanded.
Among stainless steels, austenitic stainless steel is excellent in heat resistance and workability. However, austenitic stainless steel has a large heat expansion coefficient, so if used for members which are repeatedly heated and cooled such as exhaust manifolds, heat fatigue fracture easily occurs.
On the other hand, ferritic stainless steel has a smaller heat expansion coefficient compared with austenitic stainless steel, so is excellent in heat fatigue characteristics and scale spalling resistance. Further, it does not contain Ni, so compared with austenitic stainless steel, the cost of material is low. Therefore, this is being used for general applications.
Ferritic stainless steel is lower in high temperature strength compared with austenitic stainless steel. Art for improving the high temperature strength has been developed.
As ferritic stainless steel improved in high temperature strength, for example, there are SUS430J1 (Nb steel), Nb—Si steel, and SUS444 (Nb—Mo steel). These all use solution strengthening or precipitation strengthening by addition of Nb so as to raise the high temperature strength.
Nb steel has the problem of hardening of the finished sheet, a drop in elongation, and a low r-value—an indicator of deep drawability.
Hardening of the finished sheet is a phenomenon where the presence of dissolved Nb and precipitated Nb causes hardening at ordinary temperature.
Development of the recrystallized texture is suppressed, so the elongation falls, the r-value becomes lower, and the press formability and shape freedom when forming the exhaust parts become lower.
Further, Nb is high in material cost. If added in a large amount, the manufacturing cost rises.
If excellent high temperature characteristics can be obtained by additive elements other than Nb, it would be possible to keep down the amount of addition of Nb and provide heat resistant ferritic stainless steel sheet which is low in cost and excellent in workability.
PLT's 1 to 6 disclose art relating to the addition of Cu.
In PLT 1, to improve low temperature toughness, addition of 0.5% or less of Cu is being studied.
The art which is described in PLT 2 is art which utilizes the action of Cu of raising the corrosion resistance and weather resistance.
PLT's 3 to 6 disclose the art which utilizes precipitation strengthening by Cu precipitates to improve the high temperature strength in the 600° C. or 700 to 800° C. temperature range.
These arts all require the addition of Nb. This is a problem in terms of the cost and workability.
Further, regarding improvement of the high temperature strength utilizing Cu precipitates, if the Cu precipitates are exposed to a high temperature over a long term, coarsening due to aggregation and combination of precipitate rapidly proceeds, so the precipitation strengthening ability remarkably falls.
As a result, if used for a member such as an exhaust manifold which is subjected to thermal cycles along with engine startup and stopping, long term use causes a remarkable drop in high temperature strength and the danger of heat fatigue fracture.
Further, in a system of ingredients in which Nb is added in a large amount, Cu precipitates at the coarse Laves phase and matrix phase interface at the time of high temperature heating, so the effect of precipitation strengthening by Cu precipitates is not obtained.
PLT 6 discloses the art of using composite addition of Nb—Cu—B to cause fine Cu to precipitate. However, composite precipitation with the Laves phases cannot be avoided. Furthermore, addition of a fine amount of Mo is necessary. There is a problem in workability or cost.